28 THE VASCULAR SYSTEM OF THE FROG. 



Dissect as for the veins. Pass a roll of paper down the 

 (esophagus, so as to distend it and stretch the aortic arches. 

 Clean carefidly the aortic arches, commencing at the truncus 

 arteriosus; and follow the several arteries to their distributionj 

 removing the veins arid other structures which overlie them. Note 

 the division of the truncus arteriosus in front into right and left 

 branches, each of which again divides into three aortic arches — the 

 carotid arch, the systemic arch, and the pulmo- cutaneous arch. 



I. The Carotid Arch is the most anterior of the three arches : 

 it rims round tlie side of the a3sophagus, and is connected dorsally 

 with the second or systemic arch : its chief branches are as 

 follows. 



1. The lingual artery is a small artery supplying the 



tongue. Immediately beyond the origin of the lingual 

 artery the carotid arch presents a small spongy 

 swelling, the carotid gland. 



2. The carotid artery runs round the side of the cesopha- 



o'us to its dorsal surface : it is connected with the 

 systemic arch by a short branch, the ductus Botalli, 

 which in the adult frog is usually impervious ; and 

 then turns forwards beneath the base of the skull, 

 dividing in front into the two following vessels : 

 i. The external carotid artery, supplying the roof 



and sides of the biiccal cavity, and the orbit, 

 ii. The internal carotid artery, which enters the skull 



and supplies the brain. 



II. The Systemic Arch, the middle arch of the three, runs 

 somewhat oblicpiely round the a3sophagus to the dorsal surface, 

 and unites with its fellow of the opposite side about the level of 

 the anterior ends of the kidneys to form the dorsal aorta : near 

 the level of the posterior ends of the kidneys the aorta divides 

 into the two iliac arteries. The branches of the systemic arch 

 are as follows : 



a. Branches given ofi' before the union of the two arches. 



1. The laryngeal artery is a small branch arising from the 



inner side of the systemic arch near its origin from 

 ' the truncus arteriosus, and supplying the larynx. 



2. The oesophageal arteries are one or two branches arising 



from the upi)er ])art of the arch and qntering the 

 dorsal wall of the oesophagus. 



